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(1) Field of the Invention
The invention relates to a projection device with a liquid crystal light modulator, to a liquid crystal light modulator, and to a method of manufacturing a liquid crystal light modulator.
(2) Description of Related Art
Reflective liquid crystal displays do not require a background lighting as a light source because they are constructed such that light incident from the exterior is reflected within the liquid crystal display and thus serves as a light source for the display. To reflect the light, a reflective liquid crystal display comprises reflecting electrodes. Liquid crystal displays are used as light modulators in projection devices.
A liquid crystal light modulator comprises a thin layer of liquid crystal material arranged between a cover plate and a bottom plate. The cover plate usually consists of a glass substrate which comprises a large electrode on a surface adjoining the liquid crystal material. The bottom plate usually consists of a silicon substrate which achieves a coupling to the reflecting electrodes on a surface which adjoins the enclosed liquid crystal material.
The reflecting electrodes form a matrix arrangement of pixel electrodes and are each coupled to a thin-film transistor. The thin-film transistors are switched on and off by means of trigger circuits connected to the thin-film transistors. A voltage applied across the thin-film transistor at a pixel electrode varies the direction of orientation of the liquid crystal material at the pixel electrode, and the polarization plane of the light passing through the liquid crystal is modulated. If no voltage is applied to the pixel electrode, the light passing through the light modulator either remains unchanged, i.e. if the cells comprise helical nematic liquid crystal material, or it is scattered, i.e. if the cells comprise polymer-dispersed liquid crystal material.
Since a liquid crystal light modulator is exposed to an intensive irradiation with light, the electric circuits on the silicon substrate must be protected from this light in order to avoid light-induced currents. An important component of an effective liquid crystal light modulator, therefore, is a light-reflecting layer. Usually, the light-reflecting layer comprises a metal layer and is provided between the pixel electrodes and the transistors. A contact hole is created through the light-reflecting layer for providing an electrical contact between a pixel electrode and a transistor.
A liquid crystal light modulator is known from EP 0763765 A1. An insulating layer is provided between the pixel electrode and the light-reflecting layer in this liquid crystal light modulator for electrical insulation of the pixel electrodes and the light-reflecting layer of metal. The insulating layer is structured such that it envelops the regions of a contact hole which lie in the light-reflecting layer. Since the layer thickness of the insulating layer is a few micrometers, a region of a few micrometers arises around each contact hole where the light-reflecting layer is interrupted. Light passing the voids between the pixel electrodes is reflected against the light-reflecting layer and the lower sides of the pixel electrodes and will arrive at the silicon substrate through such an interruption in the light-reflecting layer.
It is accordingly an object of the present invention to counteract the disadvantages of the prior art and to provide a projection device with an improved liquid crystal light modulator.
This object is achieved by means of a projection device fitted with a liquid crystal light modulator which comprises a semiconducting substrate on which transistor arrays and control circuits are provided, a first insulating layer provided on the substrate, a light-reflecting layer, a second insulating layer, at least two pixel electrodes, a liquid crystal unit, a cover electrode, and a front plate, wherein each pixel electrode is connected to a transistor through a contact hole which is filled with an electrically conducting material, and wherein the wall of the contact hole is fully covered with a third insulating layer.
It is advantageous if the third insulating layer has a layer thickness of between 100 nm and 500 nm.
Covering of the wall of a contact hole with an insulating layer whose layer thickness is at most 500 nm substantially reduces the size of the regions in which the light-reflecting layer is interrupted. The probability that light will pass through these regions and hit the semiconducting substrate is very small up to substantially non-existent.
It is preferred that the light-reflecting layer has at least one electrical contact to a transistor.
The electrostatic charge of the light-reflecting layer can be locally removed via the contact to a transistor.
The invention also relates to a projection device fitted with a liquid crystal light modulator which comprises a semiconducting substrate on which transistor arrays and control circuits are provided, a first insulating layer provided on the substrate, a light-reflecting layer, a second insulating layer, at least two pixel electrodes, a liquid crystal unit, a cover electrode, and a front plate, wherein each pixel electrode is connected to a transistor through a contact hole which is filled with an electrically conducting material, and wherein the wall of the contact hole is fully covered with a third insulating layer.
The invention further relates to a method of manufacturing a projection device fitted with a liquid crystal light modulator which comprises a semiconducting substrate on which transistor arrays and control circuits are provided, a first insulating layer provided on the substrate, a light-reflecting layer, a second insulating layer, at least two pixel electrodes, a liquid crystal unit, a cover electrode, and a front plate, wherein each pixel electrode is connected to a transistor through a contact hole which is filled with an electrically conducting material, in which method the wall of the contact hole is fully covered with a third insulating layer.
In contrast to the traditional manufacturing possibility for a contact hole, where a hole is etched through the first and the second insulating layer and through the light-reflecting layer, which hole is then filled with an insulating material, whereupon a second, smaller hole is etched into the insulating material and the second, smaller hole is filled with an electrically conducting material, it is possible by the method described above to create a layer on the wall of the contact hole which has clearly smaller layer thicknesses. The wide tolerances of the two photolithographic etching processes in the traditional manufacturing method have the result that the light-reflecting layer shows an interruption of a few xcexcm around each contact hole in the prior art. In the method according to the invention, however, layer thicknesses in the nm range can be achieved.
It is preferred that the third insulating layer is provided on the wall of the contact hole through deposition of the third insulating layer on the second insulating layer and on the wall as well as on the bottom of the contact hole, and those regions of the third insulating layer which lie on the second insulating layer and on the bottom of the contact hole are etched away, for which the third insulating layer on the wall of the contact hole serves as a mask.
A further advantage of this method is that the use of the third insulating layer on the wall of a contact hole as a natural mask for the etching process simplifies the method.
It may be preferred that at least one transistor is exposed through etching before the light-reflecting layer is deposited.
It is advantageous that the electrical contact can be achieved in a simple process step, and that any number of transistors not used for controlling a pixel electrode can be connected to the light-reflecting layer.